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Since the beginning of the Tertiary the sedimentology of the Gulf of Mexico Basin has been dominated by the depositional activity of the Mississippi River. The sedimentologic influence of the Mississippi diminishes with distance east or west of the Louisiana shelf, however. The Texas and northwest Florida shelf margins, for example, are characterized by a series of smaller deltas. In the inner and mid-shelf areas of these regions the near-surface sedimentary units include infilled stream channels and small deltas. Such features are commonly observed in sub-bottom seismic records from the middle and inner shelf of the northeastern Gulf, along the Apalachicola River coast of northwest Florida. The Apalachicola River is the principal source of clastic sediment to the northeastern Gulf of Mexico. During the late Holocene virtually all of the river's sediment load has been deposited in the modern Apalachicola Delta and in the river's estuary. Apalachicola Bay, which has been filling rapidly. During the Quaternary lowstands, prior to the development of the modern estuary, the river traversed the present-day inner and mid-shelf, incising a network of channels. Based on seismic records, many of these buried shelf channels were considerably larger than their modern counterparts. During lowstands the Apalachicola River also deposited coarse sediment on the shelf as deltaic and associated river-mouth sediments. These deposits comprise the modern near-surface sediments of the inner and middle shelf. An investigation of subsurface sedimentary features observed in seismic profiles provides details on the late Quaternary development of the northeastern Gulf of Mexico shelf. Seismic reflection profiles obtained on the inner and mid-shelf regions of northwest Florida reveal an approximately 50 m thickness of late Quaternary sediments, comprised of two and sometimes three discrete clastic sequences. Two lower fluvial sequences total as much as 40-50 m in thickness. A transgressive marine sand deposit overlies the older features in some places, varying in thickness form 0 to 5 m. Identification of seismic facies, combined with stratigraphic data from a suite of coastal boreholes, enables correlation of offshore seismic stratigraphic units with late Tertiary and Quaternary coastal stratigraphy.

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Claims persist in the literature alleging multiple pre-Sangamonian Pleistocene, mid-Wisconsinan, middle and late Holocene marine highstands on the northeast Gulf coastal plain. These views, still encountered even in official publications are rooted in the assumed similarity between Atlantic and northeast Gulf coastal history. A critical re-examination of the evidence is based on detailed sedimentary, microfossil, and geomorphic data from hundreds of drillholes and field sampling. Sediment data were matched with basic diagnostic criteria of depositional facies. Deposits and landforms that developed during the peak of Sangamonian transgression yielded the only evidence for higher-than-present Quaternary sea levels on the northeast Gulf. Pre-Sangamonian marine units are absent in the subsurface and not exposed in coastal plain surfaces. Post-Pliocene uplift and erosion had removed littoral and nearshore units from the northeast coastal plain. Upland ridges, mistaken for relict barriers, are elongated, high interfluves. Composed of alluvial deposits, they are bounded by semiparallel lineaments of apparently tectonic origin and incised by stream erosion. Combined with lineaments, rare covered karst depressions on a late Pleistocene alluvial plain provide the slight relief of subdued linear features that had been mistaken for relict barrier islands, associated with multiple Pleistocene highstands. Claims for wide Holocene sea level oscillations and record highstands rest on the belief, unsupported by reliable sediment data, that the upper ridge lithosomes were essentially wave-built, intertidal and directly correlatable with sea level positions. However, the ridge morphology and dimensions clearly indicate the foredune origins of discussed Florida Gulf shore strandplain ridges. Cited texture parameters and sedimentary structure types also fail to lend independent diagnostic support to the intertidal origins of the highest beach ridge intervals. Wave-cut scarps and associated supratidal narrow terraces yield no independent proof for the postulated high eustatic Holocene sea levels.

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A symposium on the topic of episodic sea-level change during the Quaternary was convened as part of the Southeastern Section meeting of the Geological Society of America on April 1-2, 1993, in Tallahassee, Florida. The symposium was organized for the purpose of examining and comparing some of the recent evidence for episodicity in sea-level change and in the geologic response to such changes. The sessions examined the evidence of Quaternary sea level history which has been gathered in recent years through geologic, paleontologic and geophysical studies in the southeastern United States region.

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The northeastern Gulf of Mexico is a tide-dominated embayment known as the Apalachee Bay. Rivers draining Florida and southern Georgia deposit sediment at the coastline. The shoreline is nonetheless irregular and relatively sediment-starved, with grass beds common. Tidal range is approximately 1 m. Rising tides flood the marsh grasses to depths ranging from several inches to 18 inches, deepening the numerous, meandering, tidal creeks that intersect the shoreline. At high tide the upper reaches of these creeks attain widths of 100 feet or more in the marshes, although in their incised channels nearer the Gulf, creek widths change much less. Falling tides expose shallow tidal creek-beds, oyster bars and mud flats. An extensive program has been undertaken to measure, on a micro-scale, changes in accretion, subsidence and sedimentation at a number of sites in intertidal areas of the Apalachee Bay coast. At each site sediment erosion tables (SET) and cryogenic coring plots have been established, with measurements of subsidence and accretion collected semi-annually, or more frequently at some sites. Additionally, lead-210 cores have been collected for long-term sedimentation rate analysis in both intertidal and subtidal environments at each site. Results indicate that the marshes in general are healthy, accreting at a rate that approximately compensates for subsidence and sea-level rise (locally 2 mm/yr). Response to periodic storms is complex, and apparently depends on the duration of the storm event.

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